Green ceramic composite and method for making such composite

ABSTRACT

A composite comprising an unfired ceramic body comprising an admixture of finely divided particles of ceramic solids and sinterable inorganic binder dispersed in a volatilizable solid polymeric binder having affixed and closely conformed to a surface of said body a flexible constraining release layer comprising finely divided particles of non-metallic inorganic solids dispersed in a volatilizable solid polymeric binder, the sinterable inorganic binder of the ceramic body being such that Penetration of said binder into said constraining release layer during subsequent firing is no more than 50 μm and said constraining release layer being affixed and closely conforming such that upon firing X-Y shrinkage of the ceramic body is reduced. Also, a method for making the composite. The flexible, constraining, release layer substantially reduces and controls green ceramic body shrinkage in the x- and y-directions during firing.

This is a division of application Ser. No. 07/591,192, filed Oct. 4,1990, now U.S. Pat. No. 5,254,191.

FIELD OF INVENTION

The invention relates to a method for substantially reducing andcontrolling planar shrinkage and reducing distortion of ceramic bodiesduring firing.

BACKGROUND OF THE INVENTION

An interconnect circuit board is the physical realization of electroniccircuits or subsystems from a number of extremely small circuit elementselectrically and mechanically interconnected. It is frequently desirableto combine these diverse electronic components in an arrangement so thatthey can be physically isolated and mounted adjacent one another in asingle compact package and electrically connected to each other and/orto common connections extending from the package.

Complex electronic circuits generally require that the circuit beconstructed of several layers of conductors separated by insulatingdielectric layers. The conductive layers are interconnected betweenlevels by electrically conductive pathways through the dielectric calledvias.

One well known method for constructing a multilayer circuit is byco-firing a multiplicity of ceramic tape dielectrics on which conductorshave been printed with metallized vias extending through the dielectriclayers to interconnect the various conductor layers. (See Steinberg,U.S. Pat. No. 4,654,095.) The tape layers are stacked in registry andpressed together at a preselected temperature and pressure to form amonolithic structure which is fired at an elevated temperature to driveoff the organic binder, sinter the conductive metal and densify thedielectric. This process has the advantage over classical "thick film"methods since firing need only be performed once, saving fabricatingtime and labor and limiting the diffusion of mobile metals which cancause shorting between the conductors. However, this process has thedisadvantage that the amount of shrinkage which occurs on firing may bedifficult to control. This dimensional uncertainty is particularlyundesirable in large, complex circuits and can result in misregistrationduring subsequent assembly operations.

Pressure sintering or hot pressing, the firing of a ceramic body with anexternally applied load or weight, is a well known method for bothreducing the porosity of and controlling the shape (dimensions) ofceramic parts. (See Takeda et al., U.S. Pat. No. 4,585,706; Kingery etal., Introduction to Ceramics, p 502-503, Wiley, 1976.) Pressuresintering of ceramic circuits in simple molds is made difficult by thetendency for the part to adhere to the mold and/or for crosscontamination to occur between the part and the mold. Further,application of a load or similar constraining force to the surface of aceramic part during burnout of the organic binder may restrict theescape of volatiles, causing incomplete burnout and/or distortion.

Copending U.S. application Ser. No. 07/466,934, discloses a method forconstrained sintering that permits escape of volatiles during burnout ofthe organic binder. A release layer is applied to the surface of theunfired ceramic body. A weight is subsequently placed on the releaselayer to reduce shrinkage in the X-Y direction. The release layerbetween the weight and ceramic body provides pathways for the volatilesto escape. If a method were established whereby ceramic circuits couldbe constrained-sintered without need for a mold, without applyingexternal loads, and without restricting the escape of volatiles duringburnout, and yet still largely eliminate dimensional uncertainty in thefinal circuit, processing steps associated with firing the circuitrywith reduced shrinkage could be simplified or eliminated. The advantagewould be greater yet if the method would permit co-firing of conductivemetallic pathways on the outer surfaces of the ceramic circuit.

Flaitz et al. (European Patent Application 0 243 858) describe threeapproaches to circumventing the aforementioned difficulties. With thefirst approach, constraint is applied only to the outer edges(periphery) of the part, providing an open escape path for volatiles andan entry path for oxygen. With the second approach, a co-extensive forceis applied to the entire surface of the piece to be sintered by eitherusing co-extensive porous platens or by application of an air-bearingforce to the surface or surfaces of the piece to be sintered. With thethird approach, a frictional force is applied to the sintering body byuse of contact sheets comprised of a porous composition which does notsinter or shrink during the heating cycle and which prohibit anyshrinkage of the substrate. The composition of the contact sheets isselected so that they remain porous during firing, do not fuse to theceramic, are thermally stable so that they will not shrink or expandduring the sintering cycle, and have continuous mechanicalintegrity/rigidity. The contact sheets maintain their dimensions duringthe sintering cycle, thus restricting the ceramic part from shrinking.After lamination of the contact sheets to the article to be sintered,sintering takes place without use of additional weights.

SUMMARY OF THE INVENTION

In its primary aspect, the invention is directed to a method forsubstantially reducing X-Y shrinkage during firing of ceramic bodiescomprising the sequential steps of

a. Providing an unfired ceramic body comprising an admixture of finelydivided particles of ceramic solids and sinterable inorganic binderdispersed in a volatilizable solid polymeric binder;

b. Applying to a surface of the unfired ceramic body a flexibleconstraining layer such that the constraining layer conforms closely tothe surface of the unfired ceramic body, the flexible constraining layercomprising finely divided particles of non-metallic inorganic solidsdispersed in a volatilizable polymeric binder, the Penetration of thesinterable inorganic binder into the constraining layer being no morethan 50 μm;

c. Firing the assemblage at a temperature and for a time sufficient toeffect volatilization of the polymeric binders from both the ceramicbody and the constraining layer, forming interconnected porosity in theconstraining layer and sintering of the inorganic binder in the ceramicbody without incurring radial bulk flow of the sintered body;

d. Cooling the fired assemblage; and

e. Removing the porous constraining layer from the surface of thesintered ceramic body.

In a second aspect, the invention is directed to a composite unfiredceramic body comprising an admixture of finely divided particles ofceramic solids and sinterable inorganic binder dispersed in avolatilizable solid polymeric binder having affixed and closelyconformed to a surface thereof a constraining layer comprising finelydivided particles of non-metallic inorganic solids dispersed in avolatilizable solid polymeric binder.

In a still further aspect, the invention is directed to a method formaking the composite unfired ceramic tape comprising the sequentialsteps of applying to at least one surface of an unfired ceramic tape aconstraining layer comprising finely divided particles of non-metallicinorganic solids dispersed in a volatilizable organic medium comprisingsolid polymeric binder dissolved in a volatile organic solvent, andremoving the organic solvent by evaporation.

PRIOR ART

EPO 87 105 868.1, Flaitz et al.

The patent is directed to a constrained sintering method which uses arestraining force in the Z-direction to prohibit X-Y distortion, camberand shrinkage during firing of a ceramic MLC substrate. Prior to firing,porous, rigid unfired ceramic, thermally stable contact sheets arelaminated to the surfaces of the ceramic article in order physically torestrict the ceramic from shrinking without the application ofadditional pressure. The contact sheets maintain their mechanicalintegrity and dimensional stability throughout the sintering cycle andthe fired sheets are removed from the substrate surface by polishing orscraping.

U.S. Pat. No. 4,521,449, Arnold et al.

The patent teaches the use of a dielectric layer of ceramic material tofacilitate sintering green ceramic sheets that contain surface vias andpad areas that are joined by indented lines and filled with a conductivemetal paste. After firing, the components are coated with a suitablemetal to make them solder-wettable for lead attachment. The inventorsrecognize the need for post-metallization to accommodate the significant(17%) substrate shrinkage and distortion that is typical for firedceramic material.

U.S. Pat. No. 4,340,436, Dubetsky et al.

The patent discloses superimposing an inert, coextensive nonadherent,removable, light weight, planar platen onto a green glass ceramiclaminate to restrict lateral X-Y shrinkage and distortion when the glasshas reached coalescent temperature during firing. The inventors reportedthat platen pressures of about 0.012 to about 0.058 lbs/in² over thelaminate produced enhanced planarity and lateral dimensional integrity.

BRIEF DESCRIPTION OF THE DRAWING

The Drawing consists of six figures.

FIG. 1 is a schematic representation of the arrangement of the variouscomponents of the invention prior to firing in which a constraininglayer is affixed to both sides of a substrate.

FIG. 2 is a schematic representation of the arrangement of the variouscomponents of the invention prior to firing in which a constraininglayer is affixed to one side of a substrate and a rigid substrate isadhered to the opposite side of the substrate.

FIG. 3 is a schematic representation of the arrangement of the variouscomponents of the invention prior to firing in which multiple ceramicparts are assembled into a monolith wherein each part has a constraininglayer adhered to opposite sides.

FIG. 4 is a schematic representation of delamination at theceramic/constraining layer interface without buckling of theconstraining layer.

FIG. 5 is a schematic representation of delamination at theceramic/constraining layer interface with buckling of the constraininglayer.

FIG. 6 is a graphical correlation of inorganic binder Penetration withbinder viscosity and wetting angle.

DETAILED DESCRIPTION OF THE INVENTION

General

The general purpose of the invention is to provide a new and improvedmethod for reducing X-Y shrinkage during the firing of ceramic bodies. Apreferred application of the invention is for fabricating ceramicmultilayer circuits using conventional conductive metallizations,including conductors, resistors and the like, and dielectric tapes insuch a manner that the circuit feature dimensions established during viapunching and printing are substantially maintained during firing. Themethod of the invention is therefore more economical in by-passing manyof the sources of dimensional uncertainty in ceramic parts and byeliminating many of the circuit development and manufacturing stepsnecessary to avoid dimensional errors and misregistration.

During the firing cycle, after volatilization of the organic binders,the inorganic components of the tape undergo sintering when heated to asufficient temperature. During sintering, the particulate-porous tapeundergoes changes in its structure which are common to porousfine-grained crystalline and non crystalline materials. There is anincrease in grain size, there is a change in pore shape, and there ischange in pore size and number. Sintering usually produces a decrease inporosity and results in densification of the particulate compact.

Central to the invention is the use of a flexible ceramic constraininglayer which is applied to the surface(s) of the ceramic circuit layers.The constraining layer serves several functions: (1) it provides auniform high friction contact layer which substantially reducesshrinkage in the plane of the sintering part; and (2) it provides anescape pathway for the volatile components of the ceramic tape prior tosintering. In certain cases, it facilitates co-firing of top surfacemetallization without incurring damage thereto.

In order for the constraining layer to effectively reduce shrinkage inthe plane of the sintering part, it is applied as a flexible layer tothe surface(s) of the unfired ceramic circuit layer(s). The flexibilityof the constraining layer enables the layer to conform closely to thetopography of the unfired ceramic surface(s). Lamination of the flexibleconstraining layer to the unfired ceramic surface(s) may be used toforce the constraining layer into even closer conformance, dependingupon the mode of application of the constraining layer. For example, theconstraining layer may be spray coated, dip-coated or rolled onto theunfired ceramic in the form of a dispersion, or it may be formulated asa flexible sheet and laminated onto the unfired ceramic. Lamination isparticularly effective in reducing the size of any gaps (flaws) betweena constraining layer and surface(s) of ceramic body.

Close conformance of the constraining layer to the ceramic part isnecessary to prevent the constraining layer from delaminating andbuckling away the from the ceramic part during sintering. During firing,as the dielectric substrate begins to shrink, the constraining layer isput into biaxial compression by the in-plane sintering strain of thedielectric part. If the compressive stress in the constraining layerreaches a critical point, the constraining layer delaminates and bucklesaway from the sintering dielectric substrate. The buckling problemgermane to the invention can be examined by analyzing elastic laminatedplates and shells after partial debonding that are subjected tocompressive loads parallel to the laminated layers. Buckling has beenanalyzed extensively in S. P. Timoshenko and G. M. Geere, Theory ofElastic Stability, 2nd Edn., McGraw Hill, New York (1961). Specificproblems of buckling in compressed films have been analyzed in A. G.Evans and J. W. Hutchinson, On the Mechanics of Delamination andSpalling in Compressed Films, Int. J. Solids Structures, Vol. 20, No. 5,pp. 455-466, (1984).

The buckling problem can be solved for one dimension (beam), twodimensions (rectangular or square geometry), and for a circulargeometry. A circular geometry is the most appropriate for the instantconfiguration and is presented here. The problem concerns a singleinterface crack or flaw parallel to the free surfaces as shown in FIG.4. The flaw is represented by a circular delamination of radius, a,which is in biaxial compression σ_(o). If the crack or flaw is ofsufficient size, the film above the crack is susceptible to buckling. Aninterfacial flaw or delamination parallel to the free surface does notdisturb the stress field since the stress field also acts parallel tothe surface. Thus, a stress concentration at the flaw or crack edge isnot induced. If the film buckles away from the substrate as shown inFIG. 5, the separation redistributes (i.e. concentrates) the stress atthe perimeter of the interface crack which induces crack extension andfailure by buckling. Conditions at the interface crack involve acombination of opening (Mode I) and shearing 30 (Mode II) stresses. Inour situation, where the film is a compressed powder, once bucklingoccurs, the shear forces at the crack tip will easily cause the powderfilm to fail since the powder is very weak in shear and tension.

The film (constraining layer) will undergo buckling if the compressivestress exceeds the critical buckling stress for the film. Theappropriate circular solution for the present case assumes fixed orclamped film edges. The critical buckling stress, σ_(c), is expressed as##EQU1## where t is the thickness of the constraining layer, a is theradius of the crack or flaw, k=14.68 for a clamped edge (k is anumerical factor determined from the Bessel function used to solve theinitial differential equation appropriate for the circular geometry), Eis the Young's Modulus of the constraining layer and ν is Poisson'sratio. Equation (1) shows that buckling will occur during the process ifa crack or flaw of a critical size is present at the interface betweenthe constraining layer and the part being sintered. Equation (1) alsoshows that the thickness t and Modulus E of the constraining layer areimportant in determining the critical buckling stress.

In practice, flaws can be generated during the application of theconstraining layer to the ceramic body substrate and during heat-up. Ifthe constraining layer is not flexible enough to conform closely to thetopography of the ceramic circuit layer(s), or if application methodsare not optimized to ensure close conformance of the constraining layerto the topography of the ceramic circuit layer(s), then a flaw or crackmay be created at the constraining layer/ceramic circuit interface.During heat-up, flaws can be generated by thermal expansion mismatchesbetween the constraining layer and the ceramic circuit substrate.Thermal expansion flaws that are not parallel to the constraininglayer/substrate interface act as additional stress concentrators.Thermal expansion effects (cracking, etc.) can sometimes be eliminatedor reduced by using a constraining layer which has a coefficient ofthermal expansion higher than the substrate, thus, putting theconstraining layer in planar compression during heat-up.

In order to facilitate removal of the constraining layer after firing,the glass from the ceramic part which is being fired must notsubstantially Penetrate or interact with the constraining layer duringthe process. Excessive Penetration of the glass into the constraininglayer is likely to inhibit the removal of the constraining layer fromthe part being fired and possibly adversely affect the properties of theceramic substrate if a large quantity of constraining material were toadhere to the final fired part. When selecting a glass composition forthe dielectric, two general requirements should be considered. First,the glass in the dielectric substrate should meet the requirements ofthe dielectric (i.e., dielectric constant, hermeticity, sinterability,etc.) and second, the composition of the glass should be such as toinhibit glass Penetration into the constraining layer. Penetrationinhibition is controlled in part by adjusting variables such as glassviscosity, wetting angle, etc. as will be discussed below.

An analysis of the flow of a liquid into porous media can be used toexamine the glass Penetration phenomena and give insight into theprocess. The analysis can be used as a guideline in selecting both glasscomposition and constraining layer composition in conjunction with theglass requirements specified for the dielectric as discussed above. Inthe following analysis, the porous medium is the constraining layer andthe liquid is the glass in the ceramic being fired.

The analysis was developed based on Darcy's Law to predict thePenetration of viscous fluids into porous beds and particularly withinthe context of the invention, the rate of Penetration dl/dt of inorganicbinder into the constraining layer defined by: ##EQU2## where D is thepermeability of the porous medium, ΔP, is the driving pressure forPenetration, l is the length of Penetration of the liquid into themedium at time t, and η_(L) is the viscosity of the liquid.

Equation (2) is valid if we assume the gradiant of pressure with respectto the Penetration direction, ∇P, is closely approximated by the changein pressure over the Penetration distance, or ##EQU3##

Taking into consideration the radius, r, of the pore channels in theporous medium, Kozeny and Carmen show in A. E. Scheidegger, The Physicsof Flow Through Porous Media, The MacMillan Co. (1960) pp 68-90, thatpermeability, D, can be expressed as:

    D=r.sup.2 (1 -ρ)/20                                    (3)

where ρ=ρ_(B) /ρ_(T) is the relative density of the porous media, ρ_(B)is the bulk density and ρ_(T) is the theoretical density.

ΔP is the driving pressure acting to force the liquid into the porousmedium and is defined as: ##EQU4## where 2γ_(LV) cos θ/r is thecapillary pressure, P_(a) is any external pressure difference (i. e.,externally applied load), γ_(LV) is the liquid vapor surface energy andcos θ is the solid liquid contact angle.

Substituting equations (3) and (4) into equation (2) and integrating thesubstituted equation gives: ##EQU5## Since no externally applied load,P_(a), is used in the invention, equation (5) can be expressed as:##EQU6##

For a given body under a constant driving pressure, the depth ofPenetration is proportional to the square root of time. Several methodsfor deriving equation (6) are presented in the literature. In thepresent invention, the porous medium is the constraining layer and theviscous liquid is the glass in the substrate being fired. In practice,the viscosity of the glass, contact angle of the glass on theconstraining layer material, porosity and pore radius of theconstraining layer, along with time, can be adjusted to give a desireddegree of Penetration. It can also be appreciated that the liquid/vaporsurface energy can be modified by sintering in more or less reactiveatmospheres. FIG. 6 is a plot of Penetration as a function of glassliquid viscosity (η_(L)) for various contact angles for t=30 min. Radius(r), porous layer density (1×ρ) and liquid/vapor surface energy (γ_(LV))can also be used to influence Penetration as mentioned above.

As shown by equation (6) and by the correlation given in FIG. 6,Penetration can be predicted from the viscosity and contact angle of theinorganic binder and thus can be controlled by the adjustment of thesetwo variables. As used herein, the term "Penetration" refers to thePenetration value of the sinterable inorganic binder component of theunfired ceramic body as determined by the above-described correlationmethod.

The constraining layer comprises finely divided particles ofnon-metallic inorganic solids dispersed in volatilizable organic mediumprepared by standard ceramic tape casting methods. The low sinteringrate and/or high sintering temperature of the inorganic solids in theconstraining layer preserves the interconnected porosity in the layer asa pathway for volatiles and other gases to escape from both the ceramicpart being fired and the constraining layer. A sintering temperaturedifferential of at least 50° C. is adequate. The assemblage is fired ata temperature and for a time sufficient to volatilize the organicbinders from both the constraining layer and the ceramic tape and tosinter the inorganic binder in the tape. Constrained sintering, whereinexternal pressure is applied to a ceramic body during firing via loadbearing rams, cannot be achieved in conventional belt furnaces. Incontrast, since external pressure is omitted in the present process,conventional firing equipment such as belt furnaces can be used. Aftercomplete sintering of the ceramic tape layers, the assemblage is cooled.The constraining layer can be subsequently removed from the surface ofthe finished part by a dusting or by ultrasonic treatment withoutaffecting or damaging either the ceramic surface of the part, or theconductive pathways.

During the sintering process, following volatilization of the organicbinders from the constraining layer and the ceramic body to be sintered,the constraining layer exists as a layer of inorganic powder.Application of the constraining layer in the form of a flexible tapeprior to firing ensures that the loose layer of powder will be evenlydistributed over the surface of the ceramic part and that theconstraining layer can conform closely to the surface of the body beingfired.

Ceramic Solids

Dielectric substrates typically comprise sintering (binder) andnonsintering (ceramic solid) phases. The composition of the ceramicsolids in a dielectric body which can be used in the invention is notitself directly critical so long as the solids are chemically inert withrespect to the other materials in the system and have the appropriatephysical properties relative to the inorganic binder component of thedielectric body. The nonsintering solids are added essentially as afiller to adjust properties such as thermal expansion and dielectricconstant.

The basic physical properties that are essential to the ceramic solidsin the dielectric body are (1) that they have sintering temperaturesabove the sintering temperatures of the inorganic binder, and (2) thatthey do not undergo sintering during the firing step of the invention.Thus, in the context of this invention, the term "ceramic solids" refersto inorganic materials, usually oxides, which undergo essentially nosintering under the conditions of firing to which they are subjected inthe practice of the invention.

Thus, subject to the above criteria, virtually any high meltinginorganic solid can be used as the ceramic solids component ofdielectric tape. For example, such materials as BaTiO₃, CaTiO₃, SrTiO₃,PbTiO₃, CaZrO₃, BaZrO₃, CaSnO₃, BaSnO₃, Al₂ O₃, metal carbides such assilicon carbide, metal nitrides such as aluminum nitride, minerals suchas mullite and kyanite, zirconia and various forms of silica. Even highsoftening point glasses can be used as the ceramic component providingthey have sufficiently high softening points. Furthermore, mixtures ofsuch materials may be used in order to match the thermal expansioncharacteristics of any substrate to which they are applied.

Inorganic Binder

The composition of the inorganic binder which can be used in the ceramicbodies for use in the invention is also not itself directly critical solong as it is chemically inert with respect to the other materials inthe system and it has the appropriate physical properties relative tothe ceramic solids in the ceramic body and the non-metallic solids inthe constraining layer.

In particular, it is essential that the Penetration of the inorganicbinder component of the ceramic body into the constraining layer duringthe firing not exceed 50 μm and preferably not exceed 25 μm. If thePenetration exceeds about 50 μm, removal of the constraining layer islikely to become difficult. Though the invention is not limited to thesetemperatures, firing will usually be conducted at a peak temperature of800°-950° C. and held at least 10 minutes at the peak temperature.

The basic physical properties that are preferred for the inorganicbinder in the ceramic body for use in the method of the invention are(1) that it have a sintering temperature below that of the ceramicsolids in the body, (2) that it undergo viscous phase sintering at thefiring temperatures used, and (3) that the wetting angle and viscosityof the inorganic binder are such that it will not penetrate appreciablyinto the constraining layer during firing.

The wetting characteristics of the inorganic binder, usually a glass,are determined by measuring the contact angle of the sintered inorganicbinder on a smooth planar surface of the inorganic solids contained inthe constraining layer. This procedure is described hereinbelow.

It has been determined that if the inorganic binder has a contact angleof at least 60°, it is sufficiently non-wetting for use in theinvention. It is nevertheless preferred that the contact angle of theglass be at least 70°. In the context of the method of the invention,the higher the contact angle, the better are the release properties ofthe constraining layer.

When, as is usual, the inorganic binder component of the ceramic unfiredtape is a glass, it may be either a crystallizing or non-crystallizingglass at the firing conditions.

The particle size and particle size distribution of the inorganic binderare likewise not narrowly critical, and the particles will usually bebetween 0.5 and 20 microns in size. It is, however, preferred that the50% point of the inorganic binder, which is defined as equal parts byweight of both larger and smaller particles, be equal to or less thanthat of the ceramic solids. Sintering rate is related directly to theratio of inorganic binder to ceramic solids and inversely to the glasstransition temperature (Tg) and particle size of the inorganic binder.

Polymeric Binder

The organic medium in which the glass and refractory inorganic solidsare dispersed is comprised of the polymeric binder, optionally havingdissolved therein other materials such as plasticizers, release agents,dispersing agents, stripping agents, antifouling agents and wettingagents.

To obtain better binding efficiency, it is preferred to use at least 5%wt. polymer binder for 95% wt. ceramic solids. However, it is furtherpreferred to use no more than 20% wt. polymer binder in 80% wt. ceramicsolids. Within these limits, it is desirable to use the least possibleamount of binder vis-a-vis solids in order to reduce the amount oforganics which must be removed by pyrolysis and to obtain betterparticle packing which gives reduced shrinkage upon firing.

In the past, various polymeric materials have been employed as thebinder for ceramic tapes, e.g., poly(vinyl butyral), poly(vinylacetate), poly(vinyl alcohol), cellulosic polymers such as methylcellulose, ethyl cellulose, hydroxyethyl cellulose, methylhydroxyethylcellulose, atactic polypropylene, polyethylene, silicon polymers such aspoly(methyl siloxane), poly(methylphenyl siloxane), polystyrene,butadiene/styrene copolymer, polystyrene, poly(vinyl pyrollidone),polyamides, high molecular weight polyethers, copolymers of ethyleneoxide and propylene oxide, polyacrylamides, and various acrylic polymerssuch as sodium polyacrylate, poly(lower alkyl acrylates), poly(loweralkyl methacrylates) and various copolymers and multipolymers of loweralkyl acrylates and methacrylates. Copolymers of ethyl methacrylate andmethyl acrylate and terpolymers of ethyl acrylate, methyl methacrylateand methacrylic acid have been previously used as binders for slipcasting materials.

More recently, Usala, in U.S. Pat. No. 4,536,535, has disclosed anorganic binder which is a mixture of compatible multipolymers of 0-100%wt. C₁₋₈ alkyl methacrylate, 100-0% wt. C₁₋₈ alkyl acrylate and 0-5% wt.ethylenically unsaturated carboxylic acid of amine. Because the polymerspermit the use of minimum amounts of binder and maximum amounts ofdielectric solids, their use is preferred with the dielectriccomposition of this invention. For this reason, the disclosure of theabove-referred Usala patent is incorporated by reference herein.

Frequently, the polymeric binder will also contain a small amount,relative to the binder polymer, of a plasticizer which serves to lowerthe glass transition temperature (Tg) of the binder polymer. The choiceof plasticizers is, of course, determined primarily by the polymer whichmust be modified. Among the plasticizers which have been used in variousbinder systems are diethyl phthalate, dibutyl phthalate, dioctylphthalate, butyl benzyl phthalate, alkyl phosphates, polyalkyleneglycols, glycerol, poly(ethylene oxides), hydroxyethylated alkyl phenol,dialkyldithiophosphonate and poly(isobutylene). Of these, butyl benzylphthalate is most frequently used in acrylic polymer systems because itcan be used effectively in relatively small concentrations.

Tape Manufacture

Unfired tapes are prepared by casting a slurry of the dielectricparticles and inorganic binder dispersed in a solution of binderpolymer, plasticizer and solvent onto a carrier such as polypropylene,Mylar® polyester film or stainless steel and then adjusting thethickness of the cast film by passing the cast slurry under a doctorblade. Thus, tapes which are used in the invention can be made by suchconventional methods, which are described in greater detail in U.S. Pat.No. 4,536,535 to Usala.

It will be understood that the unfired tapes used in the method of theinvention will frequently contain vias for electrical interconnection oflayers, registration holes and other perforations to accommodate devicesand chip attachment. It has nevertheless been found that the methodremains effective to reduce X-Y shrinkage even when the tape doescontain such perforations.

In some instances, the tape may contain fillers such as ceramic fibersto provide special properties such as thermal conductivity or tensilestrength to the fired tape. Though the invention was developed and isdescribed above primarily in the context of firing ceramic bodies madefrom layers of ceramic tape, it will be realized that the invention canalso be used to reduce X-Y shrinkage during firing of odd-shapednon-planar articles such as cast or molded ceramic parts.

Constraining Layer

The constraining layer for use in the method of the invention iscomprised of non-metallic particles dispersed in a solid organic polymerbinder. As mentioned above, it is preferred that the non-metallicparticles in the constraining layer have a lower sintering rate than theinorganic binder of the substrate being fired at the firing conditionsand that the wetting angle of the inorganic binder on the constrainingmaterial and the viscosity of the inorganic binder be such that binderPenetration into the constraining layer is within the bounds statedpreviously. Thus, the composition of the inorganic solids component ofthe constraining layer is likewise not critical as long as theabove-mentioned criteria are met. Any non-metallic inorganic materialcan be used as long as it does not undergo sintering during firing andas long as the wetting angle of the inorganic binder in the ceramic body(part) being fired on the constraining tape and the viscosity of theinorganic binder in the ceramic body are within the preferred bounds ofinorganic binder Penetration into the constraining layer as theinorganic binder undergoes sintering during the firing process. Althoughthe inorganic non-metallic solids used in the constraining layer may bethe same as those used in the ceramic body mullite, quartz, Al₂ O₃,CeO₂, SnO₂, MgO, ZrO₂, BN and mixtures thereof are preferred. However,glassy materials can be used provided their softening points aresufficiently high so that they do not undergo sintering when they arefired in accordance with the invention.

The constraining layer can be applied in the form of a flexible tape, athick film paste, spray, dip, roll, etc. Regardless of the form in whichthe layer is applied, it is essential that the layer be flexible inorder to attain close conformance to the ceramic body surface to reduceand preferably minimize, the size of any gaps (flaws) at theconstraining layer/ceramic body interface and increase the criticalstress value at the interface. In general, the same binder polymerswhich are suitable for the unfired ceramic tape will be suitable for theconstraining layer when it is applied as a tape.

As used herein, the terms "thick film" and "thick film paste" refer todispersions of finely divided solids in an organic medium, whichdispersions are of paste consistency and have a theology which makesthem capable of being applied by conventional screen printing. Otherdispersions having a consistency and rheology suitable for spray; dip orroll-coating may also be used. The organic media for such pastes areordinarily comprised of liquid binder polymer and various rheologicalagents dissolved in a solvent, all of which are completely pyrolyzableduring the firing process. Such pastes can be either resistive orconductive and, in some instances, may even be dielectric in character.Such compositions may or may not contain an inorganic binder, dependingupon whether or not the functional solids are sintered during firing.Conventional organic media of the type used in thick film pastes arealso suitable for the constraining layer. A more detailed discussion ofsuitable organic media materials can be found in U.S. Pat. No. 4,536,535to Usala.

To ensure the formation of interconnected porosity in the constraininglayer in order to provide an escape pathway for polymer decompositionproducts, the pore escape channels (void or pore structure) between theindividual particles within the constraining layer must be sufficient insize and remain open during heatup. For the pore channels to remain openduring heatup, the sintering rate of the constraining layer materialmust be less than the sintering rate of the ceramic part being fired aspreviously discussed. The pore structure in the constraining layer isdetermined by the characteristic particle arrangement or assembly withinthe layer. The arrangement or packing of particles in the layer isinfluenced by several factors including: the volume fraction of solids,the solids particle size, size distribution, and shape, the degree ofdispersion of the particles in the initial casting, the dryingcharacteristics of the casting, whether the layer is applied by dip orspray slurrying, and how the layer is applied. Furthermore, the pore orvoid structure in a tape, spray, or dip layer that contains a polymermatrix will most likely be different in the layer after the polymer ispyrolyzed. Keeping the foregoing conditions in mind, it is possible topack particles to a bulk density of ˜90 vol % solids. On the other hand,a lower limit on bulk density of ˜10 vol % solids should be practicableto provide sufficiently large pore channels without serious degradationof the X-Y compressive stress capability of the layer and withoutsignificant Penetration of the glass into the layer.

Process Variables

An essential characteristic of the method of the invention is that theconstraining layer conform closely to the surface of the substrate.Where the constraining layer is applied as a flexible sheet, closeconformance can be achieved by laminating the sheet to the unfireddielectric tape package.

The firing cycle for the method of the invention is subject to thephysical characteristics of the solids contained in both the ceramicbody and the constraining layer and is further limited by the heatingrate capability of the oven or kiln in which the materials are fired. Atypical batch furnace firing cycle which can be used for manyapplications is to heat the assemblage at the rate of 3° C. per minuteto 600° C., then 5° C. per minute to a peak temperature of 850° C.,maintaining the assemblage at peak temperature for 30 minutes, and thencooling the assemblage by turning off the furnace. In a typicalcommercial installation, the firing characteristics of the materials arechosen so that they are suitable for the performance characteristics ofthe available furnace or kiln. Firing can, of course, be conducted ineither a batch, intermittent or continuous fashion.

Upon completion of firing, the constraining layer is in the form of adry, porous layer in which the particles are held together only weaklyby van der Waals forces since during firing the organic bindervolatilizes and the particles within the layer have not sintered.Because the layer has little integral strength, it can be easily removedby brushing. The removal of the fired constraining layer is neverthelesscharacterized by the need for very little mechanical energy, andcertainly grinding is not required as it is for the prior art processesin which hot pressing is used.

The invention is frequently used to make more complex multilayer partsin which one or more of the dielectric layers has printed thereon athick film electrically functional pattern such as a resistor orconductive lines or both. When this is the case, the dielectric andelectrically functional layers can be fired sequentially or they can beco-fired. Moreover, multiple parts can be stacked vertically in a singlemonolith and cofired. In such a monolith, a constraining layer liesbetween each part and on the top and bottom of the monolith, such thateach part has a constraining layer in close conformance to the top andbottom ceramic surface as shown in FIG. 3. Whether firing a singlemultilayer part or multiple multilayer parts assembled into a monolith,the firing temperature profile and/or the components of the dielectriclayers and electrically functional layers must be selected in suchmanner that the organic media of all the layers are completelyvolatilized and the inorganic binders of the respective layers are wellsintered. In some instances, it may be necessary that the conductivephase of the thick film metallization be sintered as well. The selectionof components having these relative properties is, of course, wellwithin the skill of the thick film art.

The invention also permits the firing of multilayer parts comprisingmultiple dielectric tape layers and thick film conductive pastes on arigid prefired ceramic substrate. The layers of these parts can becofired in one step or fired sequentially, as discussed above, whilemaintaining excellent X-Y dimensional stability in the dielectriclayers.

The ability to cofire multiple layers of dielectric tape on a rigidsubstrate is attractive for several reasons. The rigid substrate if madeof a high strength material, such as alumina, provides a mechanicalsupport. The rigid substrate if made of a high thermal conductivitymaterial, such as AlN or beryllia, provides a method for removing heatfrom an electronic package. Rigid substrates made of other materials,such as Si or other dielectric materials, are also potentiallyattractive. Being able to cofire multiple layers also reduces cost byreducing the number of firing steps.

The ability to cofire dielectric tape onto a rigid substrate hasadvantages over other tape on substrate methods (TOS) since themultilayer dielectric tape portion of the package can be formed byconventional methods. The dielectric layers are cut, printed withconductors or other dielectric materials, vias filled, layers stackedand laminated by conventional multilayer methodology. The constraininglayer is then applied to the surface of the unfired dielectric tape.When using the constraining layer in a tape form, which is the preferredmethod, the constraining layer tape is laminated to the exposed surfaceof the unfired dielectric tape such that intimate contact and closeconformance is achieved between the dielectric tape and constraininglayer. The dielectric tape, rigid substrate, and constraining layer tapecan either be laminated together in one co-laminating step or laminatedsequentially. For sequential lamination the dielectric tape layers arefirst laminated to the rigid substrate and then the constraining layertape is laminated to the previously laminated rigid substrate/dielectrictape laminate. For colamination, the rigid substrate, dielectric tapeand constraining layer tape are laminated in one step. If theconstraining layer is applied in a paste or spray form, the dielectrictape and rigid substrate would first be laminated together and then theconstraining layer material applied in the proper form. Other stackingand lamination methods are possible and are obvious to those skilled inthe art.

After laminating, the entire rigid substrate, dielectric tape package,and constraining layer are fired in one step in accord with the process.Via filling is not an issue in this method.

For packages that are sequentially fired, the rigid substrate,dielectric tape, and constraining layer composite would be constructedand fired as described above, however, additional layers of dielectrictape would then be added and laminated to the already fired package. Inthis case, the previously fired rigid substrate/dielectric tape packageacts as the rigid substrate onto which the dielectric tape andconstraining layer material is applied to buildup additional layers ofdielectric tape.

Thermally conductive rigid substrates and high strength rigid substratesare very attractive for hybrid applications. An attractive configurationfor high power IC chip applications, is to put a cavity in thedielectric tape, cofire the cavity configuration onto a rigid AlNsubstrate in accord with the invention and then mount an integratedcircuit chip in the cavity directly on AlN. A lid would then be attachedover the cavity to provide hermeticity. The rigid AlN substrate providesa mechanical support and acts to remove heat from the package. Theconcept of providing cavities or walls of dielectric material into whichchips are mounted is attractive because it increases the level ofpackage integration.

The ability to cofire layers of dielectric tape on a rigid substrate islimited by the thermal expansion mismatch between the rigid substrate,dielectric tape, and constraining layer material. If the thermalexpansion mismatch between the materials of the laminated composite islarge, defects at the interface between the materials can occur duringheating which can lead to buckling. Also, for hybrid applications, themethod requires that at least one side of the rigid substrate be flat(planar), so that the tape layers can be attached to the planar surface.

DETAILED DESCRIPTION OF THE ASSEMBLY FIGURES

Three embodiments of the invention are shown in FIGS. 1-3. Theseembodiments are illustrative, not definitive, of assemblies of theinvention.

FIG. 1 is a schematic representation of an arrangement of the componentsof the method of the invention in which a flexible constraining layer isaffixed to both sides of a ceramic tape part.

Both sides of an unfired ceramic tape part 5 (with or withoutmetallization) are laminated with flexible constraining layers 3 and 3asuch that the constraining layers conform closely to the surface of thepart. The thusly laminated ceramic part can be fired in a conventionalfurnace by placing the assemblage on the furnace belt 1.

FIG. 2 is a schematic representation of an arrangement of the componentsof the method of the invention in which a flexible constraining layer isaffixed to only one side of a ceramic tape part.

A pre-fired ceramic substrate 7 (with or without metallization) and anunfired ceramic tape part (with or without metallization) 5 are alignedand colaminated. A flexible constraining layer 3 may be separatelylaminated to the exposed surface of ceramic tape part 5, or all threecomponents, i.e., the constraining layer 3, the tape part 5 and thepre-fired substrate 7 may be colaminated. The assemblage is then firedin a conventional furnace by placing the assemblage on furnace belt 1.

FIG. 3 is a schematic representation of an arrangement of the variouscomponents of the invention in which multiple (n) ceramic parts arealternated with n+1 constraining layers to form a monolith. In thisfigure, n is three. But, n can be any positive integer.

Unfired ceramic tape parts (with or without metallization) 5a, 5b and 5care aligned alternatively with flexible constraining layers 3a, 3b, 3cand 3d. The entire assemblage can be colaminated or subassemblies can belaminated to form the entire assembly. For example, ceramic tape part 5aand constraining layer 3a can be laminated. Constraining layer 3b andthe other layers of the assemblage can then be laminated in turn to thesubassembly. Alternatively, a subassembly such as ceramic tape part 5aand constraining layer 3a and 3b can be aligned and laminated. A secondsubassembly such as ceramic tape part 5b and 5c and constraining layer3c and 3d can be aligned and laminated. Then, the first subassembly andthe second subassembly can be aligned and laminated. After lamination,the assemblage or monolith is fired in a conventional furnace by placingthe monolith on furnace belt 1.

EXAMPLES Examples 1-9

The following set of experiments was conducted to show that the methodof the invention eliminates radial shrinkage (i.e. X-Y shrinkage) duringfiring and provides a means for fabricating multilayer packages withtight dimensional tolerances. The examples show the precise lineardimensional control provided by the process. Samples measured in thestudy were prepared from Du Pont Green Tape™ (dielectric constant ˜6).The technique used to measure linear dimension changes during firing isalso reviewed.

Samples were prepared by standard multilayer Du Pont Green Tape™processing techniques which included cutting blank layers of dielectrictape and laminating the layers under low temperature (e.g. 70° C.) andpressure (e.g. 3000 psi) to produce a monolithic unfired multilayerbody. In some instances, as indicated below, metal conductor pastes werescreen printed onto the tape layers prior to lamination. In someinstances, layers of constraining tape were added to the top and bottomof the multilayer stack prior to lamination. In other instances, thedielectric layers were first laminated without constraining layers. Inthese instances, the constraining layers were simply added to the topand bottom of the laminated dielectric layers, and the entire stack waslaminated an additional time to adhere the constraining layers.

The 2"×2" samples of Examples 1 through 5 in Table 1, were made fromeight 3"×3" planar blanks. For those samples where metallization isindicated, either two or six of the eight layers were screen printedwith Du Pont 6142 Ag conductor metallization, in a crosshatched testpattern. The test pattern was designed to replicate a high densityconductor pattern. In Example 5, the metal was applied to only half thesurface of each printed layer. Four layers of 3 mils thick constrainingtape were added to both the top and the bottom of each stack, for anoverall total of 16 layers of tape. All 16 layers were laminatedtogether at 3000 psi and 70° C. for 10 minutes. The samples were thencut to the 2"×2" size. The unfired constraining layer tape/circuit partswere placed on smooth, nonporous alumina setters and burned out at 275°C. for 1 hour. Without removing them from the setters, the parts werethen passed through a belt furnace and sintered at 850° C. Aftercooling, the constraining layers were removed by dusting.

The 5"×5" samples of Examples 6 through 9 in Table 1, were made fromeight 5"×5" planar dielectric blanks. In Examples 6 and 7, three layersof constraining tape were added to both the top and bottom of the stackprior to lamination. The 14 layers of tape were then laminated at theindicated pressures, at 70° C. for 10 minutes. For Examples 8 and 9, theeight dielectric layers were first laminated alone, at 3000 psi and 1000psi respectively, at 70° C. for 5 minutes. Three layers of constrainingtape were then added to both the top and bottom of each, and the 14layered tape parts were laminated a second time, for an additional 5minutes, at 70° C. and 3000 psi.

In order to precisely and accurately measure linear dimensional changesduring firing, which are in accord with the tolerances required inmultilayer packages, a photolithographic process was used to place arelatively high resolution pattern of 25 to 28 Au crosses with 1 milline widths on the surface of the blanked dielectric tape layers in asimple matrix. The dielectric layers so marked became the top dielectriclayers of each of the test parts. The matrix of crosses was examined byan automated traveling optical microscope before and after firing. Thelocations of the individual crosses within the matrix were digitized andrecorded in computer memory. Using the computer to drive a precision X-Ytable, the matrix was imaged and the linear distances between individualcrosses anywhere on the surface of a sample were calculated to anaccuracy of ±0.2 mil. A total of 300 to 378 linear dimension changeswere measured for each of the nine sample configurations listed in Table1.

Table 1 shows mean linear dimension changes, Δl/l_(o), where Δl is thechange in linear distance between two selected crosshatches as a resultof firing and l_(o) is the initial linear distance between them."Alternated" refers to the orientation of the individual tape layerswithin the sample. During doctor blade casting, particles have atendency to align themselves in the machine casting direction which hasbeen shown to affect shrinkage during firing. Thus it is often desirableto alternate the casting direction of the individual tape layers tominimize casting effects.

                  TABLE 1                                                         ______________________________________                                        Ex.                     Shrinkage                                             No.   Sample Configuration                                                                            (Δl/l.sub.o)                                                                       Std. Dev.                                  ______________________________________                                        1     2" × 2", 8 layers, alternated,                                                            0.001304   0.000291                                         no metal                                                                2     2" × 2", 8 layers, not                                                                    0.001404   0.000245                                         alternated, no metal                                                    3     2" ×2", 8 layers, alternated,                                                             0.000285   0.000401                                         two layers of metal                                                     4     2" × 2", 8 layers, alternated,                                                            -0.00017   0.000581                                         six layers of metal                                                     5     2" × 2", 8 layers, alternated,                                                            -0.00015   0.000647                                         six layers half metallized                                              6     5" × 5", 8 layers, not                                                                    0.002000   0.000265                                         alternated, no metal, 3000 psi                                                lamination                                                              7     5" × 5", 8 layers, not                                                                    0.002546   0.000360                                         alternated, no metal, 2000 psi                                                lamination                                                              8     5" × 5", 8 layers, not                                                                    0.000865   0.000337                                         alternated, no metal, 2 stage                                                 lamination, dielectric layers                                                 at 3000 psi, with constraining                                                layers at 3000 psi                                                      9     5" × 5", 8 layers, not                                                                    0.001067   0.000413                                         alternated, no metal, 2 stage                                                 lamination, dielectric layers                                                 at 1000 psi, with constraining                                                layers at 3000 psi                                                      ______________________________________                                    

The slight dimensional changes measured for these parts is largely dueto a material thermal expansion effect and a constraining layercompaction effect and is not attributed to sintering. The results showthat shrinkage during firing for a number of sample configurations isvirtually eliminated and that linear dimensions can be controlled to adegree of accuracy previously unattainable. The results also show thatsample geometry and metallization density do not affect shrinkagebehavior. For comparison, typical free sintered (i.e. not constrained)multilayer Du Pont Green Tape™ parts have a (Δ1/1_(o)) of 0.12 and astandard deviation of +0.002 where shrinkage is highly influenced bypart geometry and conductor metal density. Since the process offers suchtight dimension tolerance during processing, dimensional control is notan important issue when fabricating multilayer parts by this technique.

We claim:
 1. A composite comprising an unfired ceramic body comprisingan admixture of finely divided particles of ceramic solids andsinterable inorganic binder dispersed in a volatilizable solid polymericbinder having affixed and closely conformed to a surface of said ceramicbody a flexible constraining release layer comprising finely dividedparticles of non-metallic inorganic solids dispersed in a volatilizablesolid polymeric binder, the sinterable inorganic binder of the ceramicbody being such that Penetration of said binder into said inorganicconstraining release layer during subsequent firing is no more than 50μm and said constraining release layer being affixed and closelyconforming such that upon firing X-Y shrinkage of the ceramic body isreduced.
 2. The composite of claim 1 in which at least one surface ofthe unfired ceramic body has printed thereon an unfired pattern of thickfilm electrically functional paste.
 3. The composite of claim 2 in whichthe thick film pattern is printed on the constraining layer side of theceramic body.
 4. The composite of claim 3 in which the thick filmpattern is conductive.
 5. The composite of claim 3 in which the thickfilm pattern is a resistor.
 6. The composite of claim 2 or claim 3having both resistor and conductor patterns printed on the unfiredceramic body.
 7. A method for making the composite of claim 1 comprisingthe sequential steps ofa. applying to at least one surface of an unfiredceramic body a flexible constraining release layer such that theconstraining layer conforms closely to the surface of the ceramic bodyand reduces X-Y shrinkage of the ceramic body upon firing, the unfiredceramic body comprising an admixture of finely divided particles ofceramic solids and sinterable inorganic binder dispersed in avolatilizable organic medium comprising solid polymeric binder dissolvedin volatile organic solvent, the flexible constraining release layercomprising finely divided particles of non-metallic inorganic solidsdispersed in volatilizable solid polymeric binder, and sinterableinorganic binder of the ceramic body being such that Penetration of saidinorganic binder into said constraining release layer during subsequentfiring is no more than 50 μm; and b. removing the organic solvent byevaporation.
 8. The method of claim 7 in which the constraining layer islaminated to the surface of the ceramic body.